Influence of Steam Activation on Pore Structure and Acidity of Zeolite Beta: An Al K Edge XANES Study of Aluminum Coordination

The effect of steam activation on the aluminum coordination in zeolite NH4-beta was investigated by means of quantitative analysis of Al K edge XANES spectra. Framework tetrahedral aluminum is converted to octahedral aluminum after calcination and steaming, a process that, at the same time, removes NH3 and brings about Lewis acidity. After mild activation, this octahedral aluminum can revert to the tetrahedral state by NH3 treatment while preserving long-range ordering. This implies that structural changes are fully reversible. Intense steaming causes irreversible changes in the structure of the zeolite: some of the tetrahedrally coordinated aluminum is distorted and octahedrally coordinated aluminum is present in the zeolite. NH3 treatment of intensely steamed zeolite beta results in the reversal of all octahedrally coordinated aluminum to the tetrahedrally coordinated structure, while medium range ordering around some of the tetrahedral aluminum is lost. N2 physisorption experiments indicate that intense steaming creates clefts or mesopores in the zeolite crystallites, resulting in a decrease in the length of the diffusion pathways of the reactants. This explains, at least in part, the enhanced activity of zeolite beta in the Lewis acid (Al-)-catalyzed Meerwein–Ponndorf–Verley (MPV) reaction after intense steaming.